Different components in human serum inhibit multiplication of Cryptococcus neoformans and enhance fluconazole activity

Albumin Enhances Caspofungin Activity against Aspergillus Species by Facilitating Drug Delivery to Germinating Hyphae

The modest in vitro activity of echinocandins against Aspergillus implies that host-related factors augment the action of these antifungal agents in vivo. We found that, in contrast to the other antifungal agents (voriconazole, amphotericin B) tested, caspofungin exhibited a profound increase in activity against various Aspergillus species under conditions of cell culture growth, as evidenced by a ≥4-fold decrease in minimum effective concentrations (MECs) (P = 0. 0005). Importantly, the enhanced activity of caspofungin against Aspergillus spp. under cell culture conditions was strictly dependent on serum albumin and was not observed with the other two echinocandins, micafungin and anidulafungin. Of interest, fluorescently labeled albumin bound preferentially on the surface of germinating Aspergillus hyphae, and this interaction was further enhanced upon treatment with caspofungin. In addition, supplementation of cell culture medium with albumin resulted in a significant, 5-fold increase in association of fluorescently labeled caspofungin with Aspergillus hyphae (P < 0.0001). Collectively, we found a novel synergistic interaction between albumin and caspofungin, with albumin acting as a potential carrier molecule to facilitate antifungal drug delivery to Aspergillus hyphae.

Cryptococcal interactions with the host immune system

Opportunistic pathogens have become of increasing medical importance over the last decade due to the AIDS pandemic. Not only is cryptococcosis the fourth-most-common fatal infectious disease in sub-Saharan Africa, but also Cryptococcus is an emerging pathogen of immunocompetent individuals. The interaction between Cryptococcus and the host's immune system is a major determinant for the outcome of disease. Despite initial infection in early childhood with Cryptococcus neoformans and frequent exposure to C. neoformans within the environment, immunocompetent individuals are generally able to contain the fungus or maintain the yeast in a latent state. However, immune deficiencies lead to disseminating infections that are uniformly fatal without rapid clinical intervention. This review will discuss the innate and adaptive immune responses to Cryptococcus and cryptococcal strategies to evade the host's defense mechanisms. It will also address the importance of these strategies in pathogenesis and the potential of immunotherapy in cryptococcosis treatment.

Virulence in Cryptococcus species

Cryptococcus neoformans and Cryptococcus gattii are the cause of life-threatening meningoencephalitis in immunocompromised and immunocompetent individuals respectively. The increasing incidence of cryptococcal infection as a result of the AIDS epidemic, the recent emergence of a hypervirulent cryptococcal strain in Canada and the fact that mortality from cryptococcal disease remains high have stimulated intensive research into this organism. Here we outline recent advances in our understanding of C. neoformans and C. gattii, including intraspecific complexity, virulence factors, and key signaling pathways. We discuss the molecular basis of cryptococcal virulence and the interaction between these pathogens and the host immune system. Finally, we discuss future challenges in the study and treatment of cryptococcosis.

Combination treatment of invasive fungal infections

The persistence of high morbidity and mortality from systemic fungal infections despite the availability of novel antifungals points to the need for effective treatment strategies. Treatment of invasive fungal infections is often hampered by drug toxicity, tolerability, and specificity issues, and added complications often arise due to the lack of diagnostic tests and to treatment complexities. Combination therapy has been suggested as a possible approach to improve treatment outcome. In this article, we undertake a historical review of studies of combination therapy and also focus on recent studies involving newly approved antifungal agents. The limitations surrounding antifungal combinations include nonuniform interpretation criteria, inability to predict the likelihood of clinical success, strain variability, and variations in pharmacodynamic/pharmacokinetic properties of antifungals used in combination. The issue of antagonism between polyenes and azoles is beginning to be addressed, but data regarding other drug combinations are not adequate for us to draw definite conclusions. However, recent data have identified potentially useful combinations. Standardization of assay methods and adoption of common interpretive criteria are essential to avoid discrepancies between different in vitro studies. Larger clinical trials are needed to assess whether combination therapy improves survival and treatment outcome in the most seriously debilitated patients afflicted with life-threatening fungal infections.

Transferrin independent serum inhibition of Blastomyces dermatitidis

Human serum has been reported to inhibit the growth of several fungal pathogens. Serum inhibitory activity has predominantly been associated with transferrin-mediated iron binding. We found that serum from several animal species (human, dog, mouse and fetal bovine) inhibited the growth of the dimorphic fungal pathogen, Blastomyces dermatitidisin vitro. In initial studies, we found no correlation between the total iron-binding capacity of various sera and their inhibitory activity against B. dermatitidis yeast. In addition, we were unable to abrogate the inhibitory activity of human serum against B. dermatitidis yeast by the addition of ferric chloride (10-200 microM). We found that apo-transferrin had little or no inhibitory activity against B. dermatitidis yeast. Furthermore, serum from hypotransferrinemic mice (hpx/hpx) had inhibitory activity against B. dermatitidis yeast that was equivalent to that of normal mouse serum. These findings are consistent with our initial findings and suggest that serum inhibitory activity against B. dermatitidis yeast is transferrin independent. Although high concentrations (3.5-5 mg/ml) of lactoferrin did have inhibitory activity against B. dermatitidis yeast, these concentrations were orders of magnitude greater than those found in serum under normal physiological conditions, suggesting that lactoferrin was not likely to contribute to serum inhibitory activity against B. dermatitidis yeast. Our findings suggest that host iron-binding proteins may be relatively ineffective in innate host defense against infection with B. dermatitidis yeast.

Serum stimulates growth of and proteinase secretion by Aspergillus fumigatus

Serum contains iron-binding proteins, which inhibit the growth of most pathogenic microorganisms, including fungi. The purpose of this research was to investigate the effect of serum on growth of the opportunistic fungal pathogen Aspergillus fumigatus. Supplementing minimal essential medium (MEM) with up to 80% human serum or up to 80% fetal bovine serum (FBS) stimulated growth and increased the amount of A. fumigatus dry biomass approximately fourfold. In addition, a 100-fold increase in proteinase secretion, as measured by azocasein hydrolysis, was observed when 10% human serum or 10% FBS was added to MEM. The fungal proteinases secreted in serum-containing media were shown to degrade (3)H-labeled basal lamina proteins. The factor in serum that stimulated proteinase secretion was larger than 10 kDa and was 85% inactivated when the serum was heated for 30 min at 66 degrees C. The proportions of proteinases of each catalytic class secreted by A. fumigatus in the presence of serum were different from the proportions secreted in media containing single proteins. Proteinase secretion did not result from increased protein concentration in the medium per se because bovine serum albumin (BSA) at a concentration equivalent to the concentration of serum produced only 20% of the proteinase activity per milligram (dry weight) that was produced by FBS. Addition of BSA plus 100 microM FeCl(3) to MEM resulted in the same level of growth as addition of serum, indicating that a combination of nutritional factors in serum may stimulate growth. However, the level of proteinase secretion was still only 30% of the level observed with FBS. These data indicate that serum does not inhibit the growth of A. fumigatus and that the nutrients in serum result in high levels of proteinase secretion, potentially increasing the invasiveness of this species.

In vitro activities and postantifungal effects of the potent dolastatin 10 derivative auristatin PHE

The pentapeptide dolavaline-valine-dolaisoleuine-dolaproine-phenylalanine-methyl ester (auristatin PHE) is a derivative of the anticancer drug dolastatin 10 (dolavaline-valine-dolaisoleuine-dolaproine-dolaphenine). Broth microdilution assays with a wide variety of yeast and filamentous fungal species demonstrated the specificity of auristatin PHE for Cryptococcus neoformans and several species of Trichosporon. The duration of the postantifungal effect (PAFE) for C. neoformans was determined for exposure times ranging from 30 min to 2 h. For the derivative, a PAFE was detectable after 45 min of exposure. The effect plateaued after 1 h of exposure, with a PAFE of approximately 6.5 h at four or eight times the auristatin PHE MIC. In contrast, there was no measurable PAFE after 1 h of exposure to dolastatin 10. Human serum greatly prolonged the PAFE of auristatin PHE at eight times the MIC. Auristatin PHE arrested C. neoformans in the budding stage, possibly due to a tubulin-inhibitory action. Auristatin PHE has potential as a narrow-spectrum fungicidal agent and as a probe that can be used to study cryptococcal cell division.

Influence of human sera on the in vitro activity of the echinocandin caspofungin (MK-0991) against Aspergillus fumigatus

There have been several reports that the activity of echinocandin antifungal agents is not affected or decreased in the presence of human sera. It is known that these drugs are bound >80% in animal and human sera. The activity of the echinocandin caspofungin (MK-0991), a 1,3-beta-D-glucan synthase inhibitor, against Aspergillus fumigatus with and without human sera was studied. Conidia of A. fumigatus in microtest plate wells formed germlings after overnight culture in RPMI 1640. Caspofungin was then added with or without serum, and the germlings were incubated at 37 degrees C for 24 h. Human serum (5%) in RPMI 1640 alone did not significantly inhibit the growth of A. fumigatus in vitro. Caspofungin in RPMI 1640 exhibited dose-dependent inhibition, with concentrations of 0.1 and 0.05 microg/ml inhibiting 24.9% +/- 10.4% and 11.7% +/- 3.6%, respectively (n = 10; P < 0.01). The addition of 5% human serum to caspofungin at 0.1 or 0.05 microg/ml increased the inhibition to 78.6% +/- 5.8% or 58.3% +/- 19.2%, respectively (n = 10; P < 0.01 versus controls and versus the drug without serum). Lower concentrations of serum also potentiated drug activity. The effect of human sera was further seen when using caspofungin that had lost activity (e.g., by storage) against A. fumigatus at 0.1 microg/ml. Inactive caspofungin alone demonstrated no significant inhibition of hyphal growth, whereas the addition of 5% human serum to the inactive drug showed 83% +/- 16.5% inhibition (n = 5; P < 0. 01). The restoration of activity of caspofungin was seen at concentrations as low as 0.05% human serum. In contrast to prior reports, this study suggests that human serum acts synergistically with caspofungin to enhance its inhibitory activity in vitro against A. fumigatus.

Characterization of an anticryptococcal protein isolated from human serum

Human serum at low concentrations inhibits the growth of Cryptococcus neoformans in vitro. Fractionation of serum yielded a purified inhibitory protein with a molecular mass of approximately 81.8 kDa, a pI of approximately 6.2, and an amino acid sequence that matched that of human transferrin. The inhibitory activity and that of apotransferrin and 5% human serum were reversed by 10 microM freshly prepared FeCl(3).

Synergistic fungistatic effects of lactoferrin in combination with antifungal drugs against clinical Candida isolates

Because of the rising incidence of failures in the treatment of oropharyngeal candidosis in the case of severely immunosuppressed patients (mostly human immunodeficiency virus [HIV]-infected patients), there is need for the development of new, more effective agents and/or compounds that support the activity of the common antifungal agents. Since lactoferrin is one of the nonspecific host defense factors present in saliva that exhibit antifungal activity, we studied the antifungal effects of human, bovine, and iron-depleted lactoferrin in combination with fluconazole, amphotericin B, and 5-fluorocytosine in vitro against clinical isolates of Candida species. Distinct antifungal activities of lactoferrin were observed against clinical isolates of Candida. The MICs generally were determined to be in the range of 0.5 to 100 mg. ml(-1). Interestingly, in the combination experiments we observed pronounced cooperative activity against the growth of Candida by using lactoferrin and the three antifungals tested. Only in a limited concentration range was minor antagonism detected. The use of lactoferrin and fluconazole appeared to be the most successful combination. Significant reductions in the minimal effective concentrations of fluconazole were found when it was combined with a relatively low lactoferrin concentration (1 mg/ml). Such combinations still resulted in complete growth inhibition, while synergy of up to 50% against several Candida species was observed. It is concluded that the combined use of lactoferrin and antifungals against severe infections with Candida is an attractive therapeutic option. Since fluconazole-resistant Candida species have frequently been reported, especially in HIV-infected patients, the addition of lactoferrin to the existing fluconazole therapy could postpone the occurrence of species resistance against fluconazole. Clinical studies to further elucidate the potential utility of this combination therapy have been initiated.

Antifungal activity of cerebrospinal fluid against Cryptococcus neoformans and Candida species

The effect of human cerebrospinal fluid (CSF) on the growth of Cryptococcus neoformans and Candida species was tested in RPMI-1640. CSF alone was highly fungistatic for both yeasts and inhibited growth in a concentration-dependent manner. Unlike human serum, CSF did not collaborate with fluconazole for killing C. neoformans. Molecular sieve fractionation of CSF on a G-200 Sephadex column yielded a highly antifungal fraction with a molecular weight around 66 kDa. On SDS-PAGE this fraction migrated as a major and a minor band corresponding to the mobility of bovine serum albumin. These novel findings suggest that CSF contains a factor(s) that provides resistance to the growth of C. neoformans or Candida species.

Isolation of a human serum protein that inhibits the growth of Cryptococcus neoformans

Human serum at 5 to 10% (v/v) in tissue culture medium RPMI-1640, inhibits the growth of Cryptococcus neoformans by 80 to 93%. Serum fractionated on molecular sieve columns (Sephadex G-200) yielded an active protein fraction. This fraction at 100 micrograms protein/ml inhibited the growth of C. neoformans by 54%. When an active G-200 fraction was applied to a dye affinity column (Affi-Gel Blue) the fraction with inhibitory activity was bound by the column and was eluted with 1.4 M NaCl in 0.1 M phosphate buffer (pH 7.4). The bound fraction at 62.5 micrograms protein/ml inhibited C. neoformans growth by 82%. On native polyacrylamide gel electrophoresis (Nu-PAGE) the bound fraction migrated as a major and a minor band. Under the reducing conditions of sodium dodecyl sulfate (SDS)-PAGE the bound fraction yielded 4 prominent bands with MW ranging from 175 kDa to 45 kDa. Purification of the active Sephadex G-200 peak was achieved using an anion exchange column (DEAE-Sephacel). Protein eluted with 0.1 M NaCl had strong anticryptococcal activity (12.5 micrograms/ml, 79% inhibition), which in SDS-PAGE migrated as a single band with an approximate MW of 85 kDA. This protein appears important in natural host resistance to C. neoformans and polymorphisms or deficiencies may have epidemiologic and diagnostic relevance.

Human defenses against Cryptococcus neoformans: an update

Cryptococcus neoformans var. neoformans is an opportunistic fungal pathogen, especially in AIDS patients, and is found world-wide. On the other hand, Cryptococcus neoformans var. gatti (CN-g) is restricted to an association with two species of Eucalyptus trees. Alveolar macrophages (AM) constitute the first line of defense to Cryptococcus neoformans and offers some resistance. The inflammatory response to Cryptococcus neoformans with an influx of neutrophils and monocytes affords a second line of defense. Secretion of proinflammatory monokines by human AM is now being defined. The inflammatory phagocytes are efficient in killing Cryptococcus neoformans and offer strong resistance. T and B cell responses to infection, a third line of defense, results in production of lymphokines (IFNg, etc.) and specific antibodies. Enhancement of lymphocyte responses by IL-12 and IL-18 to Cryptococcus neoformans infection appears to be critical. Susceptibility of AIDS patients to Cryptococcus neoformans is associated with low CD4+ T cell counts and likely reduced efficacy of the second line of defense.

Amphotericin B and fluconazole affect cellular charge, macrophage phagocytosis, and cellular morphology of Cryptococcus neoformans at subinhibitory concentrations

Amphotericin B (AmB) and fluconazole (FLU) are the major antifungal drugs used in the treatment of cryptococcosis. Both drugs are believed to exert their antifungal effects through actions on cell membrane sterols. In this study we investigated whether AmB and FLU had other, more subtle effects on C. neoformans that could contribute to their therapeutic efficacy. C. neoformans cells were grown in media with subinhibitory concentrations of either AmB or FLU and analyzed for cellular charge, phagocytosis by macrophages with antibody and complement opsonins, appearance by scanning electron and light microscopies, and release of the capsular polysaccharide glucuronoxylomannan into the culture medium. Growth in the presence of either AmB or FLU resulted in major reductions in cellular charge, as measured by determination of the zeta potential. Phagocytosis studies demonstrated that exposure of C. neoformans to subinhibitory concentrations of AmB or FLU enhanced phagocytosis by macrophages. Scanning electron microscopy revealed that a large proportion of cells had an altered capsular appearance. Cells grown in medium with either AmB or FLU were smaller and released more glucuronoxylomannan into the culture medium than cells grown without antibiotics. The results suggest additional mechanisms of action for AmB and FLU that may be operative in body compartments where drug levels do not achieve the MICs. Furthermore, the results suggest mechanisms by which AmB and FLU can cooperate with humoral and cellular immune defense systems in controlling C. neoformans infections.

In vitro and in vivo efficacies of the azole SCH56592 against Cryptococcus neoformans

Multiple isolates of Cryptococcus neoformans were tested to compare the in vitro activity of a new triazole, SCH56592, with those of amphotericin B, fluconazole, and itraconazole, MICs of each drug were determined, and minimum fungicidal concentrations of SCH56592 and amphotericin B were measured. MICs of SCH56592 were lower than those of amphotericin B and fluconazole but not those of itraconazole. Minimum fungicidal concentrations of SCH56592 were lower than those of amphotericin B. SCH56592 in the presence of human serum produces an in vitro fungicidal effect for Cryptococcus neoformans. The data indicate that SCH56592 might exert fungicidal as well as inhibitory properties in vivo. On the basis of these results, SCH56592 was evaluated with a rabbit model of experimental cryptococcal meningitis; SCH56592 treatment was compared with treatment with fluconazole. Despite no detectable drug concentrations in the cerebrospinal fluid, the activity of SCH56592 against C. neoformans infection was equivalent to that of fluconazole. SCH56592 has potent in vitro activity against C. neoformans and compares favorably to treatment with fluconazole for a central nervous system infection. SCH56592 should be studied for use in humans with cryptococcal infections.

Study of the role of iron in the anticryptococcal activity of human serum and fluconazole

Anticryptococcal activity of human serum and apotransferrin in RPMI 1640 was studied in vitro. The effects of varying concentrations of FeCl3 on this activity was investigated. Possible synergy of serum and apotransferrin with fluconazole was also measured. The fungistatic activity of human serum, whether lyophilized, stored at 4 degrees C, fresh frozen or purchased from commercial sources vs. Cryptococcus neoformans was comparable. There was no significant loss of fungistatic activity after freezing and thawing the serum up to 10 times. The fungistatic activity of human serum was similar when tested in different tissue culture media with the exception of Medium 199. The addition of apotransferrin (2.0 or 0.2 mg/ml) to RPMI 1640 had an inhibitory effect on cryptococcal growth. This effect was reversed by 20 microM, of FeCl3 at both apotransferrin concentrations. By contrast, addition of FeCl3 to human serum and RPMI 1640 did not reverse inhibition of growth. Fluconazole synergized with the human serum preparations described, but not with pooled commercial serum, for fungicidal activity. Synergistic activity of fluconazole and human serum was not affected by the addition of FeCl3. Apotransferrin did not show any synergistic fungicidal activity with fluconazole.